In my case, it is because I have to be paying attention to something, with the intention of remembering it, in order to remember. I could walk down a street and not be able to tell you what color the house on the corner was, even if I had been looking right at it a moment ago, because I didn't hit the "save" button in my head. It didn't seem relevant at the time.
This got me in trouble back in kindergarten. The teacher told my mother that I didn't comprehend anything I was reading, because she had me read a story aloud then asked me questions about it. I wasn't paying attention while I was reading it, because I thought the story was boring. I was focusing more on how I sounded while I was reading it, trying to put the emphasis where I thought it should go in order to impress the teacher with my performance of it. I did all of the characters' voices a certain way, but wasn't really keeping track of what they were saying.
My mother told her, "I know she understands what she is reading, because when she reads things at home, we often discuss the stories afterward." Then my mother thought to ask her, "Did you tell her she was going to be asked questions about the story after she finished it?"
"Well, no."
"Try letting her know ahead of time, and see what happens."
So she did, and I answered everything perfectly.
I'm still like that. I have to know ahead of time that I'm supposed to focus on certain details in order to retain them. I remember the things I think about, but not always things that I experience, unless I feel strongly about something.